The present invention relates generally to head up display systems, and more particularly, to an improved head up display system having a diffusing image plane.
As the field-of-view (FOV) requirement of a head up display increases, the size of the image source tends to increase in order to keep a lower optical system magnification. Unfortunately, the image source size is desired to be as small as possible to reduce cost of the image source and associated packaging.
As image sources become smaller, the optical system needs to increase magnification and is challenged in producing a minimally distorted image. The magnification increase can produce an unacceptable sun load condition presented to the image source as well.
Sun load is a calculated energy irradiance value expressed in watts/cm.sup.2. The heating of the image source is due to the focusing of the sun's energy upon its surface. Should the magnification of the optical system be high enough to produce an excessive sun load condition the image source will be damaged.
Optical system performance also suffers in conventional systems when using small image sources. For a typical 10.degree. horizontal field-of-view (HFOV) head up display with an eye box of 73.5 mm in width, the relationship among vertical disparity error (the worst error within field-of-view and eye box), optical system f-number versus the horizontal dimension of the image source are shown in FIG. 1. As can be seen from FIG. 1, as the image width drops below 50 mm, the vertical disparity error increases quite rapidly.
A driver has the ability to tolerate vertical disparity from 1 to 9 mrad, otherwise double vision can occur. The vertical disparity error will stay about 1 mrad when the image source width is kept greater than 50 mm. The corresponding f-number of the optical system should stay above 0.8 to maintain a reasonable package size.
Therefore, the use of conventional optics, the size of the image source should be greater than 50 mm in width. A cost penalty is now realized due to the inability to use smaller than 50 mm. wide image sources (for this example).
A solution is needed to allow the use of small cost effective image sources, which also meets head up display optical performance requirements. The use of a diffusing image plane or diffuser is the solution that is provided by the present invention. It would therefore be desirable to have a head up display system employing a diffuser that eliminates problems associated with conventional designs.